Stress sensor

ABSTRACT

A stress sensor having a post ( 6 ) fixed to or integrated with the surface of an insulation substrate ( 1 ) capable of determining the direction and magnitude of a stress applied to the post ( 6 ) from changes in the characteristics of a strain gauge ( 2 ) made by a stimulus to the strain gauge ( 2 ) caused by the stress, wherein a stress to the post ( 6 ) can be converted efficiently into changes in the characteristics of the strain gauge ( 2 ). Consequently, the stress sensor has a strain gauge ( 2 )-disposed member provided with a locally-easy-to-deform portion where the strain gauge ( 2 ) is disposed. The strain gauge ( 2 ) is a resistance element ( 8 ) and is disposed on the surface of the insulation substrate ( 1 ), the insulation substrate mainly contains a resin material, and the easy-to-deform portion is preferably a thin-wall portion ( 7 ).

FIELD OF THE INVENTION

The present invention relates to a stress sensor for pointing device ofpersonal computers, multi-function/multi-direction switch of electronicdevices etc.

DESCRIPTION OF RELATED ART

The Japanese Patent Laid-open Publication No. JP-2000-267803 discloses astress sensor which has a post fixed to or integrated with the surfaceof an insulation substrate capable of determining the direction andmagnitude of a stress applied to the post from changes in thecharacteristics of a strain gauge made by a stimulus to the strain gaugecaused by the stress.

The structure of the stress gauge is shown in FIG. 6( a). Resistanceelements 22 serving as strain gauges are located on four positions alongtwo perpendicular lines on the surface of the substrate 20, wherein theperpendicular lines intersect at the center of the surface of thesubstrate 20. The strain gauges 2 are arranged equidistant along theintersection of the two perpendicular lines. The center of the surfaceof the substrate 20 is substantially equivalent to the center of thebottom of the post 30 with a squared-shaped bottom profile. Theresistance elements 22 are fixed on the substrate 20 such that each ofthe edges of the profile 30 b of the post bottom is opposite to each ofthe resistance elements 22.

FIG. 6( b) shows the operation of the strain gauge in the case when astress of X direction (i.e. any transverse direction) applies to thepost 30. FIG. 6( c) shows the operation of the strain gauge in the casewhen a stress of a Z direction (i.e. downward direction) applies to thepost 30.

In the both operations of FIGS. 6( b) and 6(c), solder 32 are fixed atthe ends of the substrate 20 through a circuit plate 31, and the stressmakes the portions of the substrate 20 corresponding to the profile 30 bof the bottom of the post flex. The resistance elements 22 located inthe portion stretch or contract due to the stress. The resistance of theresistance element 22 changes accordingly.

However, the sensitivity (i.e. output) corresponding to the stressapplied to the post 30 is small. The stress applied to the post 30cannot be efficiently converted into the change of the resistance.

One of the objects of the present invention is to provide a stresssensor wherein a stress to the post can be converted efficiently intochanges in the characteristics of the strain gauge.

SUMMARY OF THE INVENTION:

For solving the above problems, the present invention provides a stresssensor comprising a post 6 fixed to or integrated with a surface of aninsulation substrate 1. The stress sensor is capable of determining adirection and a magnitude of a stress applied to the post 6 from changesin characteristics of a strain gauge 2 made by a stimulus to the straingauge 2 caused by the stress, wherein a member with the strain gauge 2arranged thereon has a locally-easy-to-deform portion, and the straingauge 2 is arranged at the locally easy-to-deform portion.

If there is a locally-easy-to-deform portion in the member with thestrain gauges 2 arranged thereon, the stress is easily transferred tothe member with the strain gauges 2 arranged thereon and the stress iseasily concentrated at the easy-to-deform portion. Because the straingauge 2 is arranged at the easy-to-deform portion, the strain gauge canget big stimulus and the characteristics of the strain gauge 2 changeslargely. Thus, the stress to the post 6 can be efficiently convertedinto changes of characteristics of the strain gauge 2 to solve the aboveproblem.

The word “locally” means a location on the member close to the regionwhere the strain gauges 2 are arranged and an extension regionaccordingly. As shown in FIG. 1, thin-wall portion 7 is formed on theinsulation substrate 1, and the strain gauges 2 are arranged crossingthe thin-wall portion 7. The reason for defining the word “locally” into“narrow region” is in the point of maintaining the stress sensor in adesired strength. If it is only to efficiently convert a stress to thepost 6 into changes of characteristic of the strain gauges 2, the wholeregion (as shown in FIG. 1) or a part of the region surrounded by thethin-wall portion 7 (as shown in FIG. 1( b)) of the insulation substrate1 can be thin. However, with this kind of structure, in the case whenthe bottom profile of the post 6 stimulates the thin-wall portion 7 toomuch, the insulation substrate 1 is plastically deformed. The presentinvention is to eliminate this disadvantage.

According to the stress sensor of the present invention, the straingauges 2 can be formed on the surface of the insulation substrate 1 andcan be formed on the side surface of the post 6 as long as having amechanism for stimulating the resistance element 2 caused by a stress tothe post 6.

The stimulus can change the electrical characteristic of the straingauges 2. The flexing (i.e. deformation) of the side surface of the post6 or the insulation substrate 1 results in the stretching or contractingof the strain gauges 2 and the pressing of or removal of the pressing ofthe strain gauges. The stretching and contracting of the strain gauge 2are shown as FIGS. 6( b) and 6(c). The resistance of the resistanceelement becomes large due to the stretching of the resistance element 22and becomes small due to the contracting of the resistance element 22.The example for pressing or removal of the pressing of the strain gauges2 is that the strain gauges 2 are arranged between the bottom of thepost 6 and the insulation substrate 1. Due to the pressing of the straingauges 2, the strain gauges 2 and the easy-to-deform portions of themember with the strain gauges 2 arranged thereon are deformed at thesame time to generate large changes of characteristic. By removing thepressing, the characteristic returns to the status prior to thepressing.

Generally speaking, a stress sensor should comprise a control portionfor detecting and calculating electrical characteristic such as theresistance to function as a stress sensor.

The sentence “the post 6 is fixed on the surface of the insulationsubstrate 1” means that the post 6 and the substrate 1 are differentmembers and are fixed with each other via an adhesion. Moreover, thesentence “the post 6 is integrated with the substrate 1” means the post6 and the substrate 1 are formed into one body. “The bottom profile ofthe post” of the latter represents the portion corresponding to “thebottom profile of the post” of the former.

Elements that changes the electrical characteristic due to a stressapplied thereon, such as the resistance element 8 as shown in FIG. 1( b)is suitable to be the strain gauge 2. Except the resistance element 8, achip-migration resistor having a thin or thick film formed on theinsulation substrate 1 or a piezoelectrical element such aspiezoelectrical ceramic comprising PZT (lead ziconate titanate) ispreferred to be the strain gauge 2.

The easy-to-deform portion is the thin-wall portion 7 formed in theinsulation substrate 1 as shown in FIG. 1, for example. Methods offorming the thin-wall portions 7 are described as follows. The thin-wallportions 7 together with the insulation substrate 1 can be formed usinga formation-mold method. Alternatively, the insulation substrate 1 canbe cut into slots. Alternatively, the insulation substrate 1 can bepartially laser-melted to form slots. With the laser-melting method, thewidth of the thin-wall portion 7 can be narrowed to tens of micro-meterby easily adjusting the beam diameter. This method is preferred forlarge-scale production. From the point of narrowing the width of thethin-wall portion 7, the width of the thin-wall portion 7 can berestrained and the stress sensor can maintain a desired strength.

It is preferable fill the thin-wall portion 7 with a material softerthan that of the member with the strain gauges 2 arranged thereon.Because of the thin-wall portion 7, the member with the strain gauges 2arranged thereon is possible to be plastically deformed due to a stressto the post 6. In this case, since there exists a soft material, theeasy-to-deform portion is not extremely damaged and the soft materialstrengthens the thin-wall portion 7. By adjusting the filling amount,selecting places to be filled and selecting the filler material, theconverting ratio of changes of the characteristic of the strain gaugewhere a stress is applied can be adjusted. By changing the fillingstatus of the thin-wall portion 7, such as by adjusting the overflowamount for overflowing the filler and by adjusting the overflow statusof the overflow distance, the converting ratio can be adjusted.

In the case when the member with the strain gauges 2 arranged thereon ismade of ceramic, the soft material can be plastic with a strengthenedfiber. In the case when the member with the strain gauges 2 arrangedthereon is made of plastic with a strengthened fiber such as epoxy mixedwith a glass fiber, the soft material can be a material cured from anepoxy resin without a fiber or a material cured from a silicon resinpaste or other rubber material.

Moreover, the thin-wall portion 7 as shown in FIG. 1( a) is not visiblein FIG. 1( b) due to the arrangement of the thin-wall portion 7originally. For easy understanding, the thin-wall portion 7 isespecially shown in FIG. 1( b).

According to the structure of the present invention, the strain gauges 2as shown in FIG. 1( b) are arranged on the insulation substrate 1. It ispreferred that the line-shaped easy-to-deform portions (i.e. thethin-wall portion) are perpendicular to the straight lines connectingthe post 6 and the edges of the insulation substrate 1. With thisstructure (i.e. representing the line-shaped structure, hereinafter),the loss of a stress to the post 6 can be extremely decreased and thestress can be transferred to the easy-to-deform portion (i.e. thethin-wall portion 7). Thus a stress to the post 6 can be efficientlyconverted into changes of the characteristic of the strain gauges 2. Thereasons for the advantages are as followings.

The minimum region for flexing (i.e. deforming) the insulation substrate1 to function the stress sensor is the existing region of the straingauge 2 (resistance element 8). If there is no easy-to-deform portion(i.e. the thin-wall portion 7), a stress to the post 6 effects the wholeinsulation substrate 1. That is to say some region that is unnecessaryto be flexed is still flexed. The region that is unnecessary to beflexed includes the outer region of the insulation substrate 1 beyondthe strain gauges 2 (i.e. the edges of the insulation substrate 1) andthe region within adjacent strain gauges 2 in the insulation substrate1. The stress for flexing the unnecessary region cannot be detected fromthe strain gauge 2 and will become a stress loss.

In the case when easy-to-deform portion (i.e. the thin-wall portion 7)is dot-shaped rather than line-shaped, the stress for flexing theinsulation substrate 1 where no dots exist and no strain gauges 2 existwill become a stress loss. However, if the interval from dot to dot issmall, it is substantially a line-shaped and almost with no stress-loss.In this condition, the dot-shaped easy-to-deform portion has the samestructure and effects as the line-shaped one and can be regarded as aline-shaped one.

Because of the line-shaped structure, a stress loss is decreased and astress can be efficiently concentrated at the easy-to-deform portion(i.e. thin-wall portion). The forming of the line-shaped structure onthe side surface of the post 6 is achieved by forming continuous orintermittent slots around the post 6.

A modified example for the line-shaped structure is a top view of FIG.2. The strain gauges 2 are arranged at the backside of the insulationsubstrate 1 as shown in FIG. 2. As shown in FIG. 2( a), the bottom ofthe post 6 is square-shaped and each of the strain gauges 2 are arrangedat the backside of the insulation substrate 1 corresponding to each ofthe edges of the square, i.e. each of the thin-wall portions 7individually corresponds to each of the strain gauges 2 withoutconnecting to each other. FIG. 2( b) is the structure of FIG. 1( b),wherein the thin-wall portion 7 is dot-shaped and the dots close to eachother to substantially form a line-shaped structure. As shown in FIG. 2(c), the bottom of the post 6 is circular-shaped and the thin-wallportion 7 is ring-shaped. Even with this structure, the thin-wallportion 7 is substantially perpendicular to the straight linesconnecting the post 6 and edges of the insulation substrate 1. FIG. 2(d) turns the thin-wall portion 7 of FIG. 2( c) into dot-shaped and theinterval from dot to dot is small such that it can be regarded as aline-shaped structure. FIG. 2( e) turns the thin-wall portion 7 of FIG.2( c) into individual without connecting to each other.

In the case, when each of the thin-wall portions 7 individuallycorresponds to each of the strain gauges 2 without connecting to eachother, and in the case when the thin-wall portion 7 is dot-shaped with aclose dot-to-dot interval that is substantially regarded as aline-shaped structure, both of the case have a stress-loss. However, insome cases, the member with the strain gauges arranged thereon with alower strength is better. This is because the condition for maintainingthe member such as the insulation substrate 1 with the strain gauges 2arranged thereon in a desired strength and the condition for decreasinga stress loss or keeping a high output of a stress sensor are hardly tobe all satisfied at the same time. The two conditions are almostcontrary to each other. Thus, the stress sensor of the present inventionshould be designed according the two conditions.

Moreover, in the case when the stress loss can be ignored or is noconcern, such as, it is expected that the stress exceeding the requiremagnitude for stimulating the strain gauges 2, the thin-wall portion 7is preferred to be individually corresponding to each of the straingauges 2 without connecting to each other, and the thin-wall portion 7is also preferred to be dot-shaped with a close dot-to-dot interval suchthat the dot-shaped thin-wall portion 7 is regarded as a line-shapedstructure.

In the explanation to the line-shaped structure, the easy-to-deformportion of the

In the explanation to the line-shaped structure, the easy-to-deformportion of the present invention is represented by a thin-wall portion7. However, the line-shaped easy-to-deform portion is not limited to thethin-wall portion. For example, the material of the easy-to-deformportion can be different from that of the insulation substrate 1.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1( a) and 1(b) show a side view and a bottom view of a stresssensor of the present invention.

FIG. 2 shows top views of strain sensor of the present invention.

FIG. 3 shows the status of input and out of electrical signals in thestrain sensor of the present invention.

FIGS. 4( a), 4(b) and 4(c) show a side view, a bottom view and a topview of the stress sensor according to one embodiment of the presentinvention.

FIGS. 5( a), 5(b) and 5(c) show a side view prior to the embedding ofthe post, a side view of the embedding of the post and a top view of thestress sensor according to another embodiment of the present invention.

FIG. 6( a) shows the structure of a conventional stress sensor.

FIGS. 6( b) and 6(c) show the operation of the conventional stresssensor.

1 . . . insulation substrate

2 . . . strain gauge

3 . . . resistance element

5 . . . conductor

6 . . . post

7 . . . thin-wall portion

8 . . . resistance element

10 . . . terminal

12 . . . support cavity

14 . . . trimmable chip resistor

16 . . . substrate cavity

18 . . . terminal collection portion

19 . . . depression

20 . . . substrate

22 . . . resistance element

23 . . . post operation portion

24 . . . conductor

30 . . . post

30 b . . . bottom profile of the post

31 . . . circuit plate

32 . . . solder

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A stress sensor according to one embodiment of the present invention isapplied to a pointing device of a personal computer.

FIG. 4 shows a laminated plate (i.e. insulation substrate 1) of 0.8 mmthickness that is mainly made of an epoxy resin mixed with a glassfiber. Thin-wall portions 7 as shown in FIG. 4 are formed by using apress-formation mold. The thin-wall portions 7 with a thickness of 10%of the insulation substrate 1, i.e. about 80 μm are formed.

The thin-wall portions 7 in FIG. 4( a) are not visible in FIG. 4( b) dueto the arrangement of the thin-wall portions 7 originally. For easilyunderstanding, the thin-wall portions 7 are especially shown in FIG. 4(b).

Copper foils of thickness of 18 μm serving as conducting layers areattached to two sides of the insulation substrate 1. A circuit pattern(i.e. conductor 5) is formed on the laminated plate withtwo-side-copper-foil as the insulation substrate 1. Finally theinsulation substrate 1 is patterned on the surface and inside of theinsulation substrate 1, such that the resistance elements 8, thetrimmable chip resistors 14 and the terminals 10 are electricallyconnected as shown FIG. 3.

In the first step of patterning, some through holes are formed forforming electrical connection path extending from the surface to theinside of the laminated plate with two-side-copper-foil. In the secondstep of patterning, a conductivity layer is formed at inner-sidewall ofthe through holes by copper-electroless-plating andcopper-electroplating in sequence. In and after the third step ofpatterning, the conductivity layer on the surface is partially removedby photo-etching of the dry film photoresist to obtain a conductor 5. InFIG. 4, the paths from the end of the conductor 5 to the terminalcollection portions 18 are omitted in drawing. By using each of theresistance elements 8 (i.e. R1˜R4) and trimmable chip resistors 14 (i.e.Rtrim 1˜4), a bridge circuit as shown in FIG. 3 is formed as the path.The terminals (i.e. Vcc, GND, Yout, Xout) in the terminal collectionportions 18 are arranged in a predetermined interval.

Each insulation substrate 1 of one unit of the large substrate ispunched to form notches for the substrate cavities 16, support cavities12 and terminal collection portions 18 as shown in FIG. 6. The supportcavities 12 formed in the sensor portion of the substrate 1 of one unitare located as 4 apexes of a square. An intersection of the diagonals ofthe square is substantially equivalent to the center of the bottomprofile of the post 6 that is arranged later.

By using screen-printing with a resistance paste made of resin (i.e.carbon-resin), resistors 3 is formed and cured as shown in FIG. 6.Furthermore, in order to protect the resistors 13, a paste made ofsilicon resin is used for screen-printing. The paste is then cured toform a passivation film. A resistance element 8 is obtained.

The trimmable chip resistors 14 that are electrically connected witheach of the resistance elements 8 in series through conductivity 5 byusing a reflow method as shown in FIG. 3. The trimmable chip resistors14 are arranged at one surface opposite to the resistance elements 8 inthe sensor portion of the substrate 1.

After that, for adjusting the summation of the resistance of theresistance elements 8 and the trimmable chip resistors 14 that areconnected with the resistance elements 8 in series, the trimmable chipresistors 14 are trimmed by a laser. The reason why the resistors 3constructing the resistance elements 8 are not trimmed is because theresistors 3 have resin portion and the trimmed insulation substrate 1mainly made of resin will cause resistance unstable. The trimming bylaser is conducted at a very high temperature, which is unsuitable tothe resin.

The alumina-made post 6 whose bottom profile is square-shaped is fixedat each unit of the insulation substrate 1. The bottom of the post 6 isarranged on the insulation substrate 1 opposite the surface whereresistance elements 8 are arranged. The post 6 is fixed at each unit ofthe insulation substrate 1 in a manner that the center of the bottom issubstantially the same as that of the insulation substrate 1. Theassembly of the stress sensor is obtained.

The tolerance range of the shift of the post 6 is within the areasurrounded by the thin-wall portions 7. FIG. 6 shows the conventionalstress sensor whose tolerance range of the shift of the post is verysmall. This is because the maximum flexing position of the substrate 20corresponds to the bottom profile of the post 6 and the position ishighly related to the performance of the stress sensor. For this pointof view, the shift of the post 6 is some how released which is anadvantage of the stress sensor of the present invention compared to theconventional one.

The large substrate is cut and separated by disc cutter along cuttinglines (i.e. visible lines or invisible lines) on the surface of thelarge substrate into stress sensors according to the one unit of theinsulation substrate 1. In this example, the post 6 is fixed prior tothe cutting and the performance of working ability is good. This isbecause the process of installing the post 6 onto each of the insulationsubstrates 1 having a stress sensor thereon after cutting is adisadvantage in transferring and handling and is more complex comparingwith the process of the large substrate.

The stress sensor comprises strain gauges 2 located on four positionsalong two perpendicular lines on the surface of the insulation substrate1, wherein the perpendicular lines intersect at the center of effectregion for sensing of the surface of the insulation substrate 1. Thestrain gauges 2 are arranged equidistant along the intersection of thetwo perpendicular lines. The center of the surface of the insulationsubstrate 1 is substantially equivalent to the center of the effectregion for sensing and the center of the bottom of the post 6. With thisstructure, the stress sensor which has the post 6 fixed to or integratedwith the insulation substrate 1 is provided.

The stress sensor is fixed onto the frame of an electronics through thesupport cavities 12. Under a fix condition, the peripheral portion ofthe substrate 3 beyond the substrate cavities 16 deforms little evenwhen a stress is applied to the post 2 and serving as a non-deformingportion. The portion within the substrate cavities 16 deforms if astress is applied to the post 5 and serves as a deforming portion thatmakes the resistance element 8 stretch or contract. The whole region ofthe deforming portion is the “effect region for sensing” of thesubstrate 1 for sensor portion. Because the trimmable chip resistors 14are arranged at the non-deforming portions, the influence due to astress applied on the post 2 on changes of resistance is little.

In this example, the insulation substrate 1 is made of epoxy resin mixedwith a glass fiber. In other words, the insulation substrate 1 is mainlymade of resin material. The material of the insulation substrate 1 canbe replaced by ceramic such as alumina. However, in the case when athin-wall portion 7 as the example is formed in a ceramic, the ceramicis easily damaged starting from this portion. Moreover, it is difficultto form a locally-easy-to-deform portion in a ceramic material. Thus,the resin material is preferred to be the main composition of theinsulation substrate 1.

FIG. 3 shows input and output statuses of electrical signals of thestress sensor of the present invention. Four sets of resistance elementsand trimmable chip resistors construct a bridge circuit. A predeterminedvoltage is applied between the voltage terminals (Vcc)-(GND) of thebridge circuit. A Y-direction stress sensor is constructed by analyzingthe resistance element 8 (R1, 2) at the left side of FIG. 3 and thetrimmable chip resistor 14 (Rtrim1, 2) through the Y terminal (Yout). AX-direction stress sensor is constructed by analyzing the resistanceelement 8 (R3, 4) at the right side of FIG. 3 and the trimmable chipresistor 14 (Rtrim3, 4). Moreover, in the case when the top of the postis pressed downward (Z-direction), the resistance of each of theresistance element increases. This condition is different from theX-direction and Y-direction stress and can be detected.

By adding some functions with respect to the downward stress(Z-direction), the stress sensor can have multi-functions. In thisexample, the stress sensor of the present invention is used as pointingdevice of a computer, that is capable to be divided into signals forclicking the mouse. Moreover, in the case of using the stress sensor ofthe present invention to a multi-function/multi-direction switch of asmall portable machine such as a cell phone, the downward stress lastingfor a predetermined interval can correspond to an power on/off commandof the portable machine.

Whether the trimmable chip resistor 14 is to be used or not depends onthe material of construction of the resistance elements and the materialof the insulation substrate 1. For example, if the material of thesubstrate 1 for sensor portion is ceramic, in the case when the materialof the resistor 3 is metal glaze, even when the resistor 3 constructingthe resistance elements 8 is trimmed by a laser, the instability of theresistance can be negligible. In this case, the stress sensor can beconstructed without a trimmable chip resistor 14. If it is necessary touse trimmable chip resistor 14 for any other reason, the trimmable chipresistor 14 of course can be used.

Furthermore, substrate cavities 16 may be set, for example, for makingthe insulation substrate 1 flex easily and guiding the flexure of theinsulation substrate 1 towards the desired direction. With theeasy-to-deform portions such as the thin-wall portions 7, it plays bothof the above roles as the substrate cavities 16 does. A part of theprocess for opening the insulation substrate 1 (i.e. opening thesubstrate cavities 16) can be omitted, which is a merit.

(Another Embodiment)

FIG. 5 shows another embodiment of the present invention. As shown inFIG. 5, the stress sensor has a depression 19 in the insulationsubstrate 1. The depression 19 is embedded within the bottom of the post6. Strain gauges 2 are arranged at the surface of the insulationsubstrate 1 at a position corresponding to the profile of the depression19 with the insulation substrate 1 there-between. The stress sensor candetermine the direction and magnitude of a stress applied to the post 6from changes in the characteristics of a strain gauge 2 made by stimulusto the strain gauge 2 caused by the stress.

FIG. 5( a) shows the status before the bottom of the post 6 is embeddedinto the depression 19. FIG. 5( b) shows the status after the embedding.In this condition, the bottom profile of the post 6 is substantiallyequivalent to the profile of the depression 19. In the embedding status,it is preferred that no gap exists between the bottom of the post 6 andthe depression 19. If a gap exists between the bottom of the post 6 andthe depression 19, the bottom of the post 6 stimulating the bottom ofthe depression 19 will be regarded equivalent to the thin-wall portions7 of the insulation substrate 1 such that the bottom of the depression19 is plastically deformed. The stress sensor is also considered tomaintain a desired strength. In the case when an over stress is appliedon the post of the stress sensor shown in FIG. 5, the stress is sharedby the inner-sidewall of the depression 19. The over stress can berestrained from being applied on the bottom of the depression 19.Moreover, because the bottom of the post 6 is fixed in the depression 19under a condition that the bottom of the post 6 has been embedded intothe depression 19, the effects of stress-sharing can be increased.

It is known from prior art (FIG. 6) that the stress applied to the post6 is concentrated in the insulation substrate 1 at a location of thebottom profile of the post. In the stress sensor of FIG. 5, the stressis also concentrated at the profile of the depression 19 where thebottom profile of the post 6 is located. Therefore, by arranging thestrain gauge 2 at the profile of the depression 19, a stress to the post6 can be converted efficiently into changes in the characteristics ofthe strain gauge 2. Because of the above reasons, the stress sensor asshown in FIG. 5 can maintain a desired strength and can efficientlyconvert a stress to the post 6 into changes in the characteristics ofthe strain gauge 2. Thus, the insulation substrate 1 of the presentinvention is unnecessary to be flexed towards one side as theconventional one does. The distance from the bottom profile of the post6 to the ends of the insulation substrate 1 need not necessarily be aslarge as the conventional one. The stress sensor can be smaller than theconventional one (FIG. 6). These merits also exist in the stress sensoras shown in FIGS. 1, 2 and 4.

It is preferred that the material of the post 6 has a rigidity higherthan or equivalent to that of the material of the insulation substrate1. Thus, the insulation substrate 1 can be flexed easily and a stress tothe post 6 can be transferred to the strain gauge 2 with a highefficiency. In the case when the insulation substrate 1 is made of anepoxy resin mixed with glass fiber, the material of the post 6 ispreferably a ceramic such as alumina.

The material for fixing the post 6 and the insulation substrate 1 ispreferably an adhesive such as epoxy. The embedding process ispreferably implemented during the assembling process from the efficiencypoint of view. In the case when loading the post 6 on the insulationsubstrate 1 of a conventional stress sensor, the shift of the post 6causes the shift of the characteristics of the stress sensor. However,with the stress sensor as shown in FIG. 5, one need not worry about theshift of the characteristic of the stress sensor due to the shift of thepost 6. Because the arranging position of the post 6 can be determinedin advance.

INDUSTRIAL APPLICATION

According to an embodiment of the present invention, a stress sensor isprovided to efficiently convert a stress to the post into changes incharacteristics of the strain gauge. In this condition, the stresssensor can be maintained in a desired strength.

1. A stress sensor for electronic devices, having a post fixed to orintegrated with a surface of an insulation substrate capable ofdetermining a direction and a magnitude of a stress applied to The postfrom changes in characteristics of a strain gauge made by a stimulus tothe strain gauge caused by the stress, wherein the insulation substrateincludes a depression formed in one surface of the insulation substrateas a thin-wall portion, and the strain gauge is arranged at a surface ofthe insulation substrate opposite to the depression, and the straingauge is arranged crossing the depression, and the depression is filledwith a material softer than that of the insulation substrate.
 2. Thestress sensor as claim 1, comprising strain gauges located on fourpositions along two perpendicular lines on the surface of the insulationsubstrate, wherein the perpendicular lines intersect at a center of aeffect region for sensing of the surface of the insulation substrate,wherein the strain gauges are arranged equidistant along theintersection of the two perpendicular lines, and the center of thesurface of the insulation substrate is substantially equivalent to thecenter of the effect region for sensing and the center of a bottom ofthe post, and the post is fixed to or integrated with the insulationsubstrate.
 3. The stress sensor as claim 1, wherein the strain gauge isarranged at a smooth surface of the insulation substrate.
 4. The stresssensor as claim 1, wherein the strain gauge is a resistance elementproduced by a screen-printing process and has a characteristiccorresponding to a resistance thereof.
 5. The stress sensor as claim 1,wherein the insulation substrate is mainly made of a resin material, andthe post has a higher rigidity than that of the resin material.
 6. Thestress sensor as claim 1, wherein the depression is a locally thin-wallportion of the insulation substrate.
 7. The stress sensor as claim 1,wherein the depression is a line-shaped.
 8. The stress sensor as claim7, wherein the line-shaped depression is substantially perpendicular tostraight lines extending from the post to edges of the insulationsubstrate.